Method of detecting a lateral boundary of a reservoir

ABSTRACT

A method of detecting a lateral boundary of a compacting or expanding region in a subsurface formation, which method comprises determining non-vertical deformation of the earth&#39; s surface above the subsurface formation over a period of time; identifying at least on contraction area and at least one adjacent dilatation area of the earth&#39; s surface from the non-vertical deformation over the period of time; and using the contraction area and the adjacent dilatation area as an indication of a lateral boundary of the compacting or expanding region; and a method for producing hydrocarbons.

FIELD OF THE INVENTION

The present invention relates to a method of detecting a lateralboundary of a compacting or expanding region in a subsurface formation,and to a method for producing hydrocarbons.

BACKGROUND OF THE INVENTION

There is a need for technologies that allow monitoring of depletingreservoir regions during production of hydrocarbons from the reservoir.The geometric structure of a reservoir region is normally explored bygeophysical methods, in particular seismic imaging of the subsurfaceduring the exploration stage of an oil field. It is however difficult toextract precise information about fluid fill and connectivity betweendifferent reservoir regions from seismic data, because relatively smallfaults and seals are difficult to detect in seismic images.

U.S. Pat. No. 6,092,025 discloses a method for enhancing display ofhydrocarbon edge effects in a reservoir using seismic amplitude displaysbased on a delta-amplitude-dip algorithm applied to anamplitude-vs-offset data set obtained from the seismic amplitude.

Even at further stages of the development of a field, when data fromexploration, appraisal or even production wells are available, there isoftentimes uncertainty about the position of lateral edges of producingreservoir regions.

During production of hydrocarbons (oil and/or natural gas), thereservoir region is typically compacting, and this compaction can inprinciple be studied by time-lapse seismic surveying. In time-lapseseismic surveying, seismic data is acquired at least two points in time,to study changes in seismic properties of the subsurface as a functionof time. Time-lapse seismic surveying is also referred to as4-dimensional (or 4D) seismics, wherein time between acquisitionsrepresents a fourth data dimension.

A general difficulty in seismic surveying of oil or gas fields is thatthe reservoir region normally lies several hundreds of meters up toseveral thousands of meters below the earth's surface, but the thicknessof the reservoir region or layer is comparatively small, i.e. typicallyonly several meters or tens of meters. Sensitivity to detect smallchanges in the reservoir region is therefore an issue. Typicallyoperators must gather data from several years of production before cleardifferences can be detected and conclusions about reservoir propertiescan be drawn.

Similar issues arise in the case of an expansion of a subsurface region.One particular example is the expansion of a reservoir region due toinjection of a fluid into a subsurface formation, e.g. CO₂ or water.Another example involves the heating a subsurface region, in which casethe reservoir region will expand. There is a need for a more simplemethod to explore the lateral extension of a compacting or expandingregion in a subsurface formation.

SUMMARY OF THE INVENTION

To this end the present invention provides a method of detecting alateral boundary of a compacting or expanding region in a subsurfaceformation, which method comprises

determining non-vertical deformation of the earth's surface above thesubsurface formation over a period of time;

identifying a at least one contraction area and at least one adjacentdilatation area of the earth's surface from the non-vertical deformationover the period of time; and

using the at least one contraction area and the at least one adjacentdilatation area as an indication of a lateral boundary of the compactingor expanding region.

The invention is based on the insight gained by Applicant that acompacting or expanding subsurface region gives rise to a particularpattern of non-vertical (in particular horizontal) deformation at theearth's surface. The earth's surface can also be the sea floor in caseof an offshore location. A compacting or expanding reservoir gives riseto a lateral contraction area on the surface, adjacent to a dilatationarea. This signature is characteristic for a lateral edge of thereservoir. Detection of areas of contraction and dilatation can be fareasier than conducting and interpreting seismic surveys, and it is alsomore sensitive to small changes.

In one embodiment, a non-deforming intermediate area is identifiedbetween the adjacent contraction and dilatation areas, and it isinferred that the lateral boundary is located underneath thatintermediate area. In this way a good estimate of the lateral edges ofthe reservoir is obtained, without the need for complex geophysical,geomechanical and/or reservoir modelling.

It is also possible to identify an area of maximum strain gradient atthe earth's surface, and it can be inferred that the lateral boundary islocated underneath the area of maximum strain gradient.

When deformation in a particular zone on the earth's surface ismonitored, a number of dilating or contracting areas can be identified,and this is indicative of the fact that a plurality of dilating andcontracting zones are present in the subsurface formation underneath themonitored zone.

It is not uncommon that in the exploration stage of a hydrocarbon fielda plurality of candidate reservoir regions are identified in asubsurface formation, but it is not always clear whether there is fluidconnection between such individual regions. Using the present invention,connectivity can be inferred from the number of dilating or contractingareas. If all regions are connected, there will be only one contractingor expanding area on the surface in the case of contracting or expandingregions, respectively. If there is no fluid connectivity, severalcontracting and dilating areas can be distinguished at surface.

The expanding or contracting region of which the lateral boundary isidentified can form part of a larger reservoir region, of which it maynot be known whether there is fluid connectivity throughout the largerregion. In such a case the method of the invention allows to identify aflow barrier in the larger reservoir region at the lateral boundary.

Advantageously the non-vertical deformation can be interpreted using ageomechanical and/or reservoir model of the subsurface formation.

There is also provided a method for producing hydrocarbons from asubsurface formation, wherein a lateral boundary of a compacting orexpanding region in the subsurface formation is detected according tothe method of detecting a lateral boundary.

BRIEF DESCRIPTION OF THE DRAWINGS

An embodiment of the invention will now be described in more detail andwith reference to the accompanying drawings, wherein

FIG. 1 shows schematically the vertical displacement (1 a), horizontaldisplacement (1 b), and horizontal strain (1 c) in the subsurface due toa compacting subsurface region;

FIG. 2 shows schematically areas of contraction and expansion on thesurface, for three cases of compacting subsurface regions;

FIG. 3 shows calculations of the horizontal displacement and horizontalstrain on the surface, for three cases of compacting subsurface regions;

FIG. 4 shows the horizontal strain (4 a) and horizontal strain gradient(4 b) at surface above an edge of a compacting thin horizontalsubsurface region, for various ratios of width to depth of the region;

FIG. 5 shows schematically two arrangements of sensors on the sea floor.

Where the same reference numerals are used in different Figures, theyrefer to the same or similar objects.

DETAILED DESCRIPTION OF THE INVENTION

Reference is made to FIG. 1. FIG. 1 shows three pictures of a verticalcross-section through a subsurface formation 1, which is in this caseunderneath a sea bed 2. A reservoir layer 5 is present at a distanceunder the sea floor 7, which forms the earth's surface.

FIG. 1 displays the results of a geomechanical modelling of thesubsurface formation 1. The used model is based on a homogeneousisotropic linear poro-elastic half-space extending downwardly from theearth's surface, and containing a block-shaped reservoir subject to auniform reduction in pore fluid pressure. The pore pressure change wasselected to achieve a maximum of 1 m of compaction inside the reservoir.The shear modulus is 1 GPa and the Poisson's ratio is 0.25. We note thefollowing conclusions drawn from these solutions are independent of thechoice of shear modulus and Poisson's ratio.

All pictures in FIG. 1 show shading. The shading scale is given at theright hand side, and the areas of positive and negative values areindicated by “+” and “−”, respectively.

The top picture, FIG. 1 a, is shaded according to vertical displacementin response to a compaction of the reservoir, such as due to depletionby production of hydrocarbons from the reservoir through a well (notshown). Subsidence is counted as positive displacement. The strongestsubsidence is observed in the overburden 11 just above the compactingreservoir. The sea floor 7 subsides strongest above the centre of thereservoir. The example also shows uplift in the underburden 12.

The middle picture, FIG. 1 b, maps the horizontal displacement in thesubsurface formation 1 and on the sea floor 7 in the paper plane.Displacement to the right is counted positive. It was realized that avolume decrease of a subsurface reservoir does not only lead to verticalcompaction, but is typically accompanied by a horizontal contraction ofthe reservoir. The contraction is minimum in the centre and strongesttowards the lateral edges of the reservoir. As a result, contraction isalso visible on the surface (sea floor) as a deformation. Thecontraction on the surface is strongest at and above the lateral edges15,16 of the reservoir layer.

The bottom picture, FIG. 1 c, displays horizontal strain in thesubsurface formation, which is calculated as the derivative of thedisplacement in the middle picture with respect to the horizontal (x)co-ordinate in the paper plane. Dilatative strain is counted positive.It is found that the strain changes sign from compressive to dilatative,approximately above the lateral edges of the reservoir. Therefore, thepresence of adjacent contracting and dilating areas can here be detectedby determining the strain and identifying a change of sign. From thecomparison of FIGS. 1 b and 1 c it is clear, that adjacent contractingand dilating areas can also be detected by identifying an area ofmaximum horizontal deformation.

Reference is made to FIG. 2, schematically showing several situations ofcompacted reservoir regions in a subsurface formation, e.g. due to(partial) depletion.

In FIG. 2 a, a single reservoir region 11 is present in the subsurfaceformation 12 underneath the surface 13. Vertically above the reservoirregion there is an area of contraction 15, indicated by a waved line.Adjacent thereto are areas of dilatation 17 a, 17 b, indicated by dashedlines. Intermediate between the contraction and dilatation areas are, atleast within the measurement accuracy, substantially non-deforming areas18 a 18 b, indicated by solid lines and indicative of the lateral edges19 a, 19 b of the reservoir region 11 therebelow. Note that thenon-deforming areas have (near-)zero strain, but can laterally shift, asvisible for example in FIG. 1 b.

FIG. 2 b shows the situation of two laterally adjacent reservoir regions21 a 21 b, both of which are compacting due to depletion, and betweenwhich there is no fluid communication. In this example, two areas ofcontraction 25 a 25 b can be distinguished at the surface 22, separatedby an area of dilatation 26, which is an indication at surface that thetwo reservoir regions are not in fluid communication with each other.Further areas of dilatation 27 a, 27 b and intermediate non-deformingareas 28 a, 28 b, 28 c, 28 d can also be distinguished. The intermediateareas are again indicative of the lateral boundaries 29 a, 29 b, 29 c,29 d, of the reservoir regions. It can be the case that the reservoirstructure shown in FIG. 2 b is not distinguishable in seismic imagingfrom the single reservoir region 1, because the boundaries 29 b and 29 cmerely present a narrow flow barrier between compartments of a largerreservoir structure.

FIG. 2 c shows a somewhat similar situation to that of FIG. 2 b;however, in this case the reservoir regions 31 a and 31 b are in fluidcontact with each other, as indicated by the long dashed line 32. Thetwo reservoir regions behave similar to a single region duringdepletion, so the signature of contracting and dilating areas on surfaceis similar to that of FIG. 2 a. A single contracting area 35 issurrounded by dilating areas 37 a, 37 b, with intermediate non-deformingareas 38 a and 38 b therebetween.

Reference is now made to FIG. 3, which displays quantitative examples ofthe horizontal deformation at the earth's surface induced by reservoircompaction due to depletion, for different cases of connectivity withinthe reservoir region. Calculations were made for a reservoir that is 9km wide, 100 m thick, and located 1 km below the earth's surface, andfurther using the same model assumptions as discussed for FIG. 1. Inthis example, the third reservoir dimension in the horizontal plane isequal to the horizontal dimension shown. Results are shown for a linepassing above the centre of the reservoir.

Deformation is shown for a depletion corresponding to uniform depletionequivalent to a maximum of 1 m of reservoir compaction.

In FIG. 3 a there is uniform depletion throughout the reservoir 41.Crosses 43 denote horizontal surface displacements D_(h); positivedisplacements are oriented towards the right. The maximum absolutedisplacement is found approximately at the lateral edges 45,46 of thereservoir. The solid curve 48 denotes horizontal strain; positive straincorresponds to dilatation (elongation). The strain exhibits azero-crossing at the maximums of the absolute displacement, i.e. wherethere is a transition from contraction to dilatation.

In FIG. 3 b, the reservoir 51 has a flow barrier 52 preventing fluidcommunication between the left half 53 and the right half 54. It isassumed that fluid is produced through a well (not shown) extending fromsurface into the left half 53. The right half does not deplete due tothe flow barrier 52. This can be detected at surface. The horizontaldeformation 56 and horizontal strain 58 have a signature correspondingto only the left half of the reservoir region compacting. The flowbarrier 52 is detected as the right lateral edge of the compactingregion 53.

In FIG. 3 c, finally, the reservoir region 61 has three compartments 62,63, 64. The central reservoir compartment 63 does not deplete due toflow barriers 66, 67. These can again be detected by the characteristicsignature of the horizontal deformation 68 and the horizontal strain 69at the earth's surface above, at the hand of the transition betweencontracting and dilating deformation.

The assumed compaction in this example of 1 m is very substantial, andso is the magnitude of the deformation at the earth's surface. Thedeformation scales proportional to the amount of compaction. It shall beclear that much smaller effects such as compaction of the order of 1-5cm, or even less can be detected, by detecting horizontal deformation inthe same order of magnitude at surface, over distances of the order of akilometre or more.

FIG. 4 a shows the horizontal strain ε_(xx) as a function of thedistance from the centre of a depleting block-shaped region. Thehorizontal distance is normalised by the half-width of the block suchthat its lateral boundary always occurs at x=1. In all cases the regionis thin compared to its lateral extent, i.e. has a thickness of lessthan 20% of its width. Results are shown for the range of horizontalblock sizes of 20% (curve 71 a), 40% (72 a), 60% (73 a), 80% (74 a) and100% (curve 75 a) of their depth below the earth's surface. In all casesa transition from contraction above the depleting reservoir todilatation beyond the lateral edge is seen. The location of zerohorizontal strain separating regions of contraction and elongation is agood indication of the lateral edge; however, it can be seen that itonly correctly locates the edge of the depleting reservoir if thelateral extent of this region, w, is large compared to its depth belowthe earth's surface, z, i.e. w/z>>1.

FIG. 4 b shows the horizontal derivative of the horizontal strain,dε_(xx)/dx, with the curves 71 b, 72 b, 73 b, 74 b, 75 b derived fromcurves 71 a, 72 a, 73 a, 74 a, 75 a of FIG. 4 a. The horizontalderivative is maximum at the lateral boundary of the depleting reservoirregardless of its lateral extent or depth. Therefore, locating a maximumof the strain gradient on the earth's surface is an even more accurateapproach to determining the lateral edge. A derivative of strain such asthe horizontal derivative of horizontal strain is referred to as straingradient, in particular lateral strain gradient along the surface is ofinterest.

In practice, measurements will have a finite accuracy so that a zerostrain, within the measurement accuracy, can be found in a certain areaintermediate between contracting and dilating areas.

FIG. 4 also shows that edges of subsurface regions with a large w/zratio can be better detected than smaller regions. The minimum lateralsize of region detectable depends the precision of measurementsavailable for the horizontal components of deformation induced at theearth's surface. The size of this seabed horizontal strain signaldepends on the change in reservoir thickness and on the ratio of thelateral size of the reservoir to its depth.

Contraction corresponds to negative strain, and therefore maximumcontraction corresponds to the local minima in the value of straininduced at the surface. The maximum magnitude of horizontal contractionof the earth's surface due to compaction of the reservoir isapproximately equal to u/(3 πd), where u is reservoir compaction inmeters and d is the depth of the reservoir in meters.

The ratio of maximum horizontal elongation to maximum horizontalcontraction of the earth's surface for a unit compaction (1 m) is1+3πd/w, where w is the width of the depleting reservoir.

In the Figures a compacting reservoir has been discussed. It will beclear that the case of an expanding subsurface region has an inverse(qualitatively a change of sign), but otherwise analogous, signature.

Examples will now be discussed which show how the non-verticaldeformation of the earth's surface can be determined.

On land, known geodetic methods and equipment can be used, for examplesatellite based measurements such as geodetic use of global positioningsatellite systems (e.g., GPS), Laser ranging to satellites, syntheticaperture radar interferometry from orbit, but also more traditionalgeodetic techniques such as levelling, precision tilt meters and/orgravity measurements.

An important application of the present method is also in conjunctionwith offshore production of hydrocarbons, and in order to apply thepresent method at an offshore location, the deformation of the sea flooris to be measured.

In one embodiment, determining non-vertical deformation of the sea floorcomprises selecting a plurality of locations on the sea floor anddetermining the change in distance between at least one pair of thelocations over the period of time. At each such location a sensor can beinstalled, permanently or periodically, and the distance between a pairof sensors at an initial time and at a later point in time can becompared. Preferably sensors are arranged in a grid or along a line.This allows mapping of displacements in a monitoring zone on the seafloor, and also distance measurements from one location to a pluralityof other locations.

The expression ‘sensor’ is used herein to refer to any device used indetermining a change of its location, and includes for example acoustic,electric or electromagnetic transmitters, receivers, transceivers,transponders, transducers; tilt meters, pressure gauges, gravity meters,etc.

The distance can for example be determined by means of acoustictransmitters/receivers placed at the plurality of locations, or by meansof a fibre optic strain sensor coupled at a plurality of locations tothe sea floor.

It can be advantageous to measure vertical displacement of the seafloorover the same period of time. In particular, depth sensors such aspressure or gravity sensors can be arranged at the same locations as formeasuring non-vertical displacement. In case the vertical displacementis available as well, a relationship such as a ratio between horizontaland vertical displacements at a selected point, or more points ifavailable, can be determined and used to estimate the lateral positionof a centre of compaction or expansion in the subsurface formation.

In FIGS. 5 a and 5 b two arrangements of a measurement network on thesea floor are sketched. At each location 31 an acoustic transmitterand/or receiver is arranged, suitably a transponder responding by anacoustic signal to a signal it receives from another transponder.Suitable acoustic transponders are for example manufactured by SonardyneInternational Limited of Yateley, UK, and these are typically used forpositioning of equipment on the sea floor.

By a linear arrangement as in FIG. 5 a, an extended one-dimensionalhorizontal displacement profile can be measured, as e.g. in FIG. 1 or 3.The grid of FIG. 5 b allows mapping of the displacement in twodimensions. Also, distances from one of the locations 31 to severalnearest neighbours and further neighbours can be determined, whichallows to carry out consistency checks so as to increase the overallaccuracy of measurements. Of course other grids are possible as well,and it is not required to adhere to a regular grid. More or lesstransponders can be installed.

A suitable distance between locations of adjacent transponders on thesea floor is from 10 to 100% of the reservoir depth, preferably between20 and 60%, such as 40% of reservoir depth.

Using a pair of acoustic transponders an acoustic travel time can bedetermined, which can be converted to a distance between the respectivelocations using the speed of sound in sea water. Preferably, sound speedsensors are arranged on the sea floor as well, such as one at eachtransducer location, to be able to take fluctuations due to e.g.temperature or salinity changes into account, thereby increasingaccuracy of the measurements.

Subsea transponders preferably operate wireless and are suitablyequipped with a power supply such as batteries that allows extendedoperation of many months, preferably at least 6 months, more preferablyseveral years. Data can be stored for days, weeks or months, andtransmitted to a transducer on a buoy, ship, or platform. Because theunderlying deformation is slow, in the order of few cm/year at maximum,an acoustic transducer network does not need to operate continuouslywhich saves battery life. The transponders can be permanently installed,but also periodical installation at pairs of locations is possible,carried out by a remotely operated vehicle for example. A permanentinstallation is preferred, however, since repositioning errors arecircumvented in this way. This is in fact an advantage of sub-seaacoustic lateral measurements over subsidence measurements by pressuresensors, which have insufficient long-term stability for accuratemeasurements in a permanent installation over periods of months, andneed therefore regular calibration for which they need to be removedfrom the sea floor.

Alternatively, fibre optic strain sensors can be used for measurement ofthe non-vertical sea-floor deformation. Such sensors are for examplemanufactured by Sensornet Ltd. of Elstree, UK. A fibre optic strainsensor can monitor strain over extended distances of kilometres, and astrain profile with a resolution of about 1 m can be obtained. Thesensor cable is to be anchored to the sea floor to provide sufficientcoupling.

Another measurement option is through repeated imaging, such as sonarimaging, from moving vehicles with precise positioning.

Advantageously, vertical displacement may be monitored as well. In oneembodiment involving a sea floor installation for monitoringdeformation, sensors for detecting vertical displacement such aspressure and/or gravity sensors may be included. It becomes clear fromFIG. 2 that complementary information can be obtained from horizontaland vertical displacement. For example, the maximum horizontaldisplacement is observed above the lateral edges of the reservoir, andthe ratio of vertical to horizontal displacement is a very sensitiveindicator of the centre of the compacting or expanding reservoir, asvertical displacement is maximum there and horizontal displacementsubstantially zero.

1. A method of detecting a lateral boundary of a compacting or expanding region in a subsurface formation, the method comprising: determining non-vertical deformation of the earth's surface above the subsurface formation over a period of time; identifying at least one contraction area and at least one adjacent dilatation area of the earth's surface from the non-vertical deformation over the period of time; and using the at least one contraction area and the at least one adjacent dilatation area as an indication of a lateral boundary of the compacting or expanding region.
 2. The method according to claim 1 wherein a near-horizontal component of the deformation of the earth's surface is determined.
 3. The method according to claim 1, wherein the at least one contraction area and the at least one adjacent dilatation area at the earth's surface are separated by a non-deforming intermediate area, and wherein it is inferred that the lateral boundary is located underneath the non-deforming intermediate area.
 4. The method according to claim 1, wherein an area of maximum strain gradient is identified at the earth's surface, and wherein it is inferred that the lateral boundary is located underneath the area of maximum strain gradient.
 5. The method according to claim 1, wherein a number of contraction areas and adjacent dilatation areas in a predetermined zone on the earth's surface is determined, and wherein it is inferred using the number whether there is more than one expanding or compacting region in the subsurface formation.
 6. The method according to claim 5, wherein the method further comprises distinguishing a plurality of regions in the subsurface formation, at least one of which changes its volume due to production of a fluid from or injection of a fluid into that region; inferring from the number of contraction areas or adjacent dilatation areas whether there is fluid connectivity between the regions.
 7. The method according to claim 1, wherein the expanding or contracting region of which the lateral boundary is identified forms part of a larger reservoir region, and wherein a flow barrier in the larger reservoir region is identified at the lateral boundary.
 8. The method according to claim 1, wherein the non-vertical deformation at the earth's surface is interpreted using a geomechanical model of the subsurface formation.
 9. A method for producing hydrocarbons from a subsurface formation, wherein a lateral boundary of a compacting or expanding region in the subsurface formation is detected according to the method of claim
 1. 